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Fires in informal settlements are a major risk facing big cities within the developing countries, not only concerning life safety but also because they exert social and economic pressures on these communities. One of the cities most affected by informal settlement fires is Cape Town, Western Cape, South Africa. A research project, namely IRIS-Fire, was launched in order to develop a fundamental understanding of the technical issues regarding fire spread within the informal settlements of the Western Cape. In early investigations it was found that one of the main fire spread mechanisms between dwellings is external flaming and the heat flux from the vertical openings of the burning dwelling to its surroundings. In this paper a computational study is reported which seeks to quantify the effect of adding horizontal roof openings to the design of these informal dwellings. The main parameters in the design of the computational simulations were determined based on surveys done in the informal settlements in the Western Cape. Twenty different cases were investigated using different assumed fuel loads as 270, 410, 550 and 690 MJ/m2 and horizontal opening sizes of 0.04, 0.16, 0.32 and 0.64 m2. It was found that: adding horizontal openings with sizes larger than 0.16 m2 can significantly increase the time for the dwelling to reach flashover, reduce the heat flux from external flaming to the surroundings, and reduce the size of the external flame plume.